Kerf and yield: the arithmetic behind the choice

What kerf is

Kerf is the width of the slot a saw cuts. The portal's yield article defines it as the saw plate thickness plus the set of the teeth. Every millimetre of it becomes sawdust, so kerf is one of the largest single sources of loss and the main technical argument in the contest between saw types.

The kerf share formula

The share of material lost per cut for a board of thickness t is:

kerf share = kerf / (t + kerf)

Worked example with a 1 inch (25.4 mm) board, using the example kerfs from a USDA Forest Service article (USA):

The source's own figures of 21% and 12% differ slightly because it uses a different board thickness. The formula ignores planing allowance and drying shrinkage.

A second example, from a Polish test (2023) on a frame saw and a horizontal band saw. Frame saw: 2.2 mm plate plus 0.9 mm set per side gives roughly 4.0 mm. Band saw: 1.0 mm plate plus 0.75 mm set per side gives roughly 2.5 mm. For boards about 26.5 mm thick:

• Frame saw: 4.0 / 30.5 = about 13%.
• Band saw: 2.5 / 29.0 = about 9%.

The kerf sums depend on reading "set per side" as both sides; the portal flags this as an interpretation.

What the yield gains look like

• Older US figures: in the 1920s only about 40% of a log became primary product; mid-1990s circular mills averaged about 50% conversion and band mills about 57%. These come from an advocacy article for thin kerf and are illustrations, not benchmarks.
• A 1962 study cited by Steele (1984, North America) found that cutting kerf from 12/32 to 9/32 inch raised board-foot yield by about 7% on logs 5.5 to 12 inches in diameter.
• In Steele's worked example an 8.3 inch log gained 8.3% when kerf fell from 0.240 to 0.180 inch, but not from an extra board: the smaller kerf allowed a pattern change to wider lumber.

So kerf savings are discontinuous. They show up when they tip the log into a better sawing pattern.

Thinner is not always better

Thomas and Buehlmann (USA, 2022) simulated red oak logs with kerfs of 0.09 to 0.20 inch. For an 18 inch log, cutting kerf from 0.16 to 0.12 inch gave a significant volume gain of 7.4 board feet but did not significantly increase the number of boards, which needed 0.11 inch. Because kerfs are already thin, further reductions give smaller gains than in earlier decades.

Narrow kerf also costs stiffness. In a laboratory frame-saw resawing test (Europe, 2022), narrow-kerf blades (1.4 mm kerf, 0.9 mm thickness) had a static stiffness of 19.4 N/mm against 60.8 N/mm for 2.9 mm kerf blades, and a critical feed force of 493 N against 3640 N. That is roughly 3 times less stiffness and 7 times lower critical force. Thin blades deflect more and produce dimensional error. This was a small laboratory test showing a mechanism, not an industrial rule.

Checklist: before switching to a thinner kerf

• Can guides, tension and feed hold the thinner blade without more thickness variation?
• Will the smaller kerf change the sawing pattern, or is the gain just a few extra millimetres of sawdust saved?
• Have you measured your current kerf (plate plus set) and your current thickness variation?
• Is the claimed gain from your species, log size and product, or from a different setting?

Small samples cannot show a kerf effect. The Polish test used six logs, and its yield difference (about 72.6% against 69.7% in the text) was not statistically significant; the paper is also internally inconsistent between abstract, text and tables.

Key points

• Kerf = plate thickness + set; the share lost per cut is kerf / (t + kerf).
• A wide circular kerf wastes roughly twice the share of a thin band kerf on a 1 inch board (about 23% against 12%).
• Kerf gains are discontinuous and show up when they enable a better pattern.
• Narrower blades lose stiffness; thin kerf pays only if the line can hold the blade steady.
• Older and advocacy figures illustrate orders of magnitude and are not mill targets.